Method and system using wavelength division multiplexing for eliminating and reducing light diffusion and light reflection interference in interference path

ABSTRACT

A method using wavelength division multiplexing for reducing the light diffusion and light reflection interference in an interference path, comprising: connecting a wavelength division multiplexer ( 10 ) serially at the end of a sensing optical fiber ( 6 ); using the wavelength division multiplexer ( 10 ) to extract a wavelength component from a working path for measuring the interfering signal caused by light diffusion and light reflection; using the signal as a reference, extracting the effective signal component that has been interfered with by light diffusion and light reflection, and obtaining a pure effective signal. Because the device connected at the end of the sensing optical fiber ( 6 ) is passive and requires no power, the system is easy to implement and is particularly suitable for situations in which power provision is difficult at the end of the sensing optical fiber ( 6 ). The method is suitable for long distance pipeline monitoring and wide-range optical fiber perimeter security. Also provided is a system using wavelength division multiplexing for reducing the light diffusion and light reflection interference in an interference path.

TECHNICAL FIELD

The present invention belongs to the field of optical fiber sensing technology, in particular eliminate the impact of backscattered light in an optical fiber sensor.

BACKGROUND

Optical fiber sensing technology is often used in large-scale, long-distance monitoring, such as security monitoring used in oil pipelines, high-voltage power grids, pipelines, communications cable and other infrastructure, which the fiber used to be the sensor, real-timely acquiring related disturbance signal, determine the location of the disturbance occurred by the analysis characterize. The structure of single core feedback optical path is: using a single fiber as sensing fiber, the fiber itself is not closed, only apply a feedback device at the end of the fiber, such as a mirror constituting interference optical path. In practice, this structure laying is flexible. The characteristics of such monitoring systems is: light carrying the disturbance information transmitted to the end of the fiber, then reflect by feedback device.

The following is a positioning technology of single core feedback positioning system.

As shown in FIG. 1, we use a sensing section for the optical fiber (optical cable). 1 is the start point of the optical fiber (optical cable), and 2 is a feedback device at the end of the sensing section, such as a mirror. The incident light retrace through the feedback device. Suppose there is a disturbance at point D outside, modulation of light phase is φ(t), when the light twice perturbed points D, phase modulation is subject to:

φ₁(t)=φ(t)+φ(t−T)

wherein, T=2n_(eff)L/c, L is the distance between disturbance point D and feedback device 2, c is the speed of light in vacuum, n_(eff) is the effective refractive index of the optical fiber.

As shown in FIG. 2, we configure an interference optical path.

Interference optical path include the following parts: N*M (N, M are integers) coupler 3, P*Q (P, Q are integers) coupler 4, optical fiber delayer 5(delay τ), an optical fiber (optical cable) 6, and feedback device 2. 3 a 1, 3 a 2, . . . , 3 aN, 3 b 1, 3 b 2 are ports of coupler 3, 3 a 1, 3 a 2, . . . , 3 aN are co-rotating ports with a total of N, 3 b 1, 3 b 2 are two ports in another group co-rotating ports (with a total of M) of coupler 3. 4 a 1, 4 a 2, 4 b 1 are ports of coupler 4, 4 a 1, 4 a 2 are two ports in a group co-rotating ports (with a total of P) of coupler 4, 4 b 1 are two ports in another group co-rotating ports (with a total of Q) of coupler 4. Optical fiber 6 is induction optical fiber. Feedback device 2 make the light transmitted along the fiber go back to the fiber 6 and return to the coupler 4. Light source input through the port 3 a 1 of coupler 3, after splitting in coupler 3, output respectively through the port 3 b 1, 3 b 2, two optical paths is:

□: 3 b 1→5→4 a 1→4 b 1→6→2→6→4 b 1→4 a 2→3 b 2

II: 3 b 2→4 a 2→4 b 1→6→2→6→4 b 1→4 a 1→5→3 b 1

The two optical paths join at coupler 3 again and generate interference, interference signals output respectively through port 3 a 1, 3 a 2, 3 aN.

In the interference optical path, the light firstly enter delayer 5 and then enter fiber cable 6, the phase modulation applied to the light is:

φ₂(t)=φ(t−τ)+φ(t−τ−T)

Phase difference between two coherent interference lights is:

Δφ=[φ(t)+φ(t−T)][φ(t−τ)+φ(t−τ−T)]

In the spectrum of phase difference, there is a frequency drop point, or “notch point”, and we can determine the location of the disturbance arising according to the notch point. “Notch point” is shown in FIG. 3, in this amplitude—frequency diagram obtained by time frequency transform, the “O” mark the notch point. The relationship between notch point and disturbance position is:

${{f_{null}(k)} = {\frac{k}{2} \cdot \frac{c}{2n_{eff}L}}},\left( {{k = {{2n} - 1}},{n \in N}} \right)$

wherein, ƒ_(null) (k) is frequency of k-order notch point.

We can see from the above principle, the coherent light must transmit from the endpoint 1 of sensing optical fiber 6 to endpoint 2 and then return to sensing optical fiber 6, in order to carry the position “L” message. However, in practice, due to the structural characteristics of the optical fiber and the fiber itself defects and other reasons, there is a scattered light in optical fibers, such as Rayleigh scattering light and the like.

As shown in FIG. 4, 7 is a scatter point, backscattered light along the optical cable go back to interference structure, and therefore there is two beams:

I: 3 b 1→5→4 a 1→4 b 1→6→7→6→4 b 1→4 a 2→3 b 2

II: 3 b 2→4 a 2→4 b 1→6→7→6→4 b 1→4 a 1→5→3 b 1

Because of similar spectral characteristics, the optical path are equal without disturbance, and therefore join at the coupler 3 again will also occur interference. Obviously, the information carried by the two beam of interference light is the length L₇ between point 7 and disturbance point D. 8 is another scattering point, the length information carried by the interference formed by backscatter is the length L₈ between point 8 and disturbance point D, apparently, L₇≠L₈≠L, since these interference is mixed at the output, the interference light generated by Brillouin backscattered light or Raman backscattered light can be filter out by optical filter, but for the interference light generated by Rayleigh scattering light, or the interference light generated by contact point of optical path, it is impossible to eliminate by optical filtering method, will affect the purity of useful interference signal, and will directly affect the accuracy of the disturbance L position. Generally, the intensity of interference generated by backscattered light, contact reflected is significantly less than the intensity of interference generated by reflected light (effective interference signal), and will not have a significant impact on the effective interference signal, accuracy of L can meet the actual needs. But after the monitoring circuit reach a certain length, scattered light affects the entire line obviously, then we can observe the obvious interference signal distortion has occurred, the system can not obtain a valid interference signal normally. Acquired signal contains not only effective interference signal and further contains spurious interference signal caused by scattering light.

Similarly, reflection by the contact point of optical path can also cause the same adverse effects on the interference signal.

The impact of scatter (reflect) light in the conventional path is not only the obvious restriction in monitoring system. When a large scatter (reflect) point exists, the system can not be properly tested in the line.

In order to cut the impact of the signal, the invention 201010508357.2 (FIG. 5) is proposed by use phase generated carrier technology to separate effective interference phase information from optical output mixed with backscattered light, contact point reflected light interference signal to obtain pure signal having effective disturbance position information, so as to eliminate the impact of back scattered light and the like purposes. In the technology, at the end of sensing optical fiber (optical cable) 6, access a phase modulator 9 close to the feedback device 2, apply modulation signal to phase modulator 9. The backward transmitting light scattered(reflected) by scatter(reflection) point (such as scatter points 7, 8) does not reach the feedback device 2 at the end of the sensing optical fiber 6. The signals are not modulated because of without passing through the phase modulator 9. The effective light reflected by the feedback device 2 reach the end and pass through the phase modulator 9, so the signal is modulated to sideband of fundamental frequency or double frequency of the modulation frequency which scatter interference signal can not reach and separated with scattered light signals. The extraction of effective light information can be achieved by corresponding signal processing means, so as to avoid the interference of the scattered light.

Since the described technique connects the phase modulator 9 series with the end of the sensing optical fiber 6, the optical path is phase-modulated and electrical signal is applied to the modulation signal, therefore the means connected to the end of sensing optical fiber is an active device that requires power. It is difficult to provide power to the end of sensing optical fiber, therefore the application of the method is limited.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a method to provide no power to the end of sensing optical fiber and eliminate the impact of backscattered light in optical fiber sensor.

The present invention use wavelength division multiplexing technology to obtain the interference signal produced by independent scattered light, so as to obtain the pure active signal from the interference output signal affected by interference signal. The present invention is improvement to the system shown in FIG. 5, connect WDM (wavelength division multiplexer) series with the end of the sensing optical fiber, WDM will be used to separate a wavelength component from the working path for measuring interference signal produced by scattered light, the signal is used as a reference to extract effective signal components interference by the scattered light to get clean valid signal. This method involves a simple structure, since the end of the fiber without inducing increased active device, structure of this method is simple and easier to implement. Specific methods are as follows.

The basic interference optical path structure of the system of the present invention is shown in FIG. 4. In the original optical path structure shown in FIG. 5, a phase modulator 9 is connected at the end of the sensing optical fiber 6 and close to the feedback device 2, but in the present invention a WDM 10 is series connected between the end of the sensing optical fiber 6 and the feedback device 2, wherein the WDM 10 is provided with a multiplexed port 10 a and two splitting ports 10 b, 10 c; the corresponding wavelengths of these two splitting ports 10 b, 10 c are λ1, λ2 respectively; The multiplexed port 10 a is connected with the end of the sensing optical fiber 6; the splitting port 10 b is connected with the feedback device 2 and the end of the splitting port 10 c is no use. Optical connect structure is shown in FIG. 6.

In the above optical structure, when the light with wavelength λ1 is transmitting along the sensing optical fiber 6 to the end of the feedback device 2, a part of the light is affected by scatter (reflection) point and return in advance, the remaining light is passing through the WDM 10 and reaching the feedback device 2, and then returning along the same route; therefore, the signal P₁ fromed by wavelength λ1 can be expressed as:

$\begin{matrix} {P_{1} = {P_{eff} + {\sum\limits_{i}\; {p_{S}\left( {\lambda_{1},i} \right)}}}} & (1) \end{matrix}$

wherein, P_(eff) is a signal formed by reflecting by the feedback device 2, p_(S) (λ₁, i) is a signal formed by light with wavelength λ1 caused by the i-th scatter (reflection) point of the sensing optical fiber 6,

$\sum\limits_{i}\;.$

is the sum of all scatter points along the sensing optical fiber 6 and before the WDM 10. when the light with wavelength λ2 is transmitting along the sensing optical fiber 6 to the end of the feedback device 2, a part of the light is affected by scatter (reflection) point and return in advance, the remaining light is passing through the WDM 10 and leaking from the port 10 c; therefore, the signal P₂ fromed by wavelength λ2 can be expressed as:

$\begin{matrix} {P_{2} = {\sum\limits_{i}\; {p_{S}\left( {\lambda_{2},i} \right)}}} & (2) \end{matrix}$

Wherein p_(S)(λ₂, i) is a signal formed by light with wavelength λ2 caused by the i-th scatter (reflection) point of the sensing optical fiber 6.

When λ1 is close to λ2:

$\begin{matrix} {{p_{S}\left( {\lambda_{1},i} \right)} \approx {p_{S}\left( {\lambda_{2},i} \right)}} & (3) \\ {{\sum\limits_{i}\; {p_{S}\left( {\lambda_{1},i} \right)}} \approx {\sum\limits_{i}\; {p_{S}\left( {\lambda_{2},i} \right)}}} & (4) \end{matrix}$

Thus, the signals P₁, P₂ are formed by wavelength λ1, λ2, if use a certain signal processing means, for example, adaptive algorithms, etc., the interference signal component can be removed and get an effective signal P_(eff).

The light injected into the optical fiber optical path structure of the present invention can be provided by an independent source, or the light source may be a combination of two or more different wavelengths of light through the WDM; WDM can have two splitting ports or a plurality of splitting ports.

Advantage of the present invention is that it can effectively eliminate the impact of backscattered(reflected) light in single core optical fiber sensing light path, the useful information is extracted from the signal of serious disturbances, which significantly improves the measurement of distance, enhance adaptability to the line of interference measurement system. The technology uses a WDM to obtain interference signal formed by scatter light, so the structure of this method is simple and easier to implement. Because the method use the passive components, the advantage of provide no power to sensing optical fiber end connector of a single feedback optical fiber sensing structure is maintained, especially for the location which difficult to provide power, so as to have a wider range of adaptability. The free extending layout of the end of the monitor optical fiber is easier to be achieved.

Distributed optical fiber line monitoring system of the invention can be widely used in long distance monitoring of safety monitoring in the field of telecommunications lines, power transmission lines, gas pipelines, oil pipelines, border; also be used for safety monitoring of large buildings such as dams, tunnels, mines, etc.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 is a positioning schematic diagram of single core feedback sensor;

FIG. 2 is a diagram of single core feedback interference structure;

FIG. 3 is a spectrum of the phase signal demodulated from interference signal, “O” is frequency “notch point.”;

FIG. 4 is a schematic diagram of impact by backscattered light;

FIG. 5 is a diagram of light path connection method which use phase generated carrier technology to eliminate the impact of backscattered;

FIGS. 6 is a diagram of light path connection method which use WDM technology to eliminate the impact of backscattered;

FIGS. 7 is a concrete construction which the method of the present invention may be implemented.

REFERENCE NUMERAL

1: end of the sensing optical fiber 6, 2: feedback device, 3: for the N*M (N, M are integers) coupler, 4: P*Q (P, Q are integers) coupler, 5: optical fiber delayer, delay τ, 6: sensor optical fiber (optical cable) and feedback device 2 constituted, 3 a 1, 3 a 2, . . . , 3 aN, 3 b 1, 3 b 2: port of coupler 3, 3 a 1, 3 a 2, . . . , 3 aN: co-rotating ports with a total of N, 3 b 1, 3 b 2: two ports in another group co-rotating ports (with a total of M) of coupler 3. 4 a 1, 4 a 2, 4 b 1: ports of coupler 4, 4 a 1, 4 a 2: two ports in a group co-rotating ports (with a total of P) of coupler 4, 4 b 1: two ports in another group co-rotating ports (with a total of Q) of coupler 4. 7, 8: scattering point in optical fiber, 9: phase modulator. 10: WDM connected to the end of the sensing optical fiber, 10 a: multiplexed port, 10 b, 10 c: splitting port, 11: WDM, 11 a: multiplexed port, 11 b, 11 c: splitting port; 12: WDM, 12 a: multiplexed port, 12 b, 12 c: splitting port; 13: WDM, 13 a: multiplexed port, 13 b, 13 c: splitting port.

Embodiment

The measurement system of the embodiment use interference structure shown in FIG. 2. Coupler 3 uses average of 3*3 Optical Fiber tapered single mode coupler. Coupler 4 uses average of 2*2 Optical Fiber tapered single mode Coupler. WDM is a three-port device and have two splitting ports, the wavelengths of the two splitting ports are 1310 nm and 1550 nm respectively. 11 is a WDM. 11 a is a multiplexed port. 11 b and 11 c are splitting ports. 12 is a WDM. 12 a is a multiplexed port. 12 b and 12 c are splitting ports. 13 is a WDM. 13 a is a multiplexed port. 13 b and 13 c are splitting ports. Optical input using two wavelengths of light, respectively λ1=1310 nm wavelength and λ2=1550 nm, respectively connected to ports 11 b, 11 c of WDM 11. Lights go through the port 11 a is injected into the port 3 a 1 of coupler 3. The light output from the ports 3 a 2, 3 a 3 are injected into the corresponding port 12 a, 13 a of WDM 12, 13, corresponding lights with 1310 nm components output from 12 b, 13 b, corresponding lights with 1550 nm components output from 12 c, 13 c. Lights output from the port 12 c, 13 c are interference signals generated by scatted (reflected) light, and lights output from the port 12 b, 13 b are active interference signal affected by signals generated by scatted (reflected) light.

Light source is S03-B type super radiation diode (SLD) produced by 44 research institute of the Institute of Industrial Electronics Group Corporation. Coupler 4 uses average of 2*2 Optical Fiber tapered single mode Coupler. Both of them are produced by Wuhan Research Institute of Posts and Telecommunications. Fiber used by fiber delayer is G652 single-mode fiber. Photoelectric converter used in photoelectric conversion and information processing is GT322C500 of InGaAs photodetector produced by 44 research institute. Feedback device 2 is produced by optical fiber end steamed aluminized production, reflectance greater than 95%. WDM 10 is a FBT single-mode device.

In the single core sensing path, an active joint connection point is 10 km from end of sensing optical cable 6 (feedback device 2), at which point reflection>2 dB, disturbance applied near the port 4 b 1 to sensor cable 6. If do not use this The method of the invention, the system can not properly positioned. After use the modulation and demodulation method, the system can locate accurately. 

1. A method using wavelength division multiplexing for eliminating and reducing light diffusion and light reflection interference in interference path, characterized by: a WDM (10) is series connected between the end of a sensing optical fiber (6) and a feedback device (2) in the conventional interference optical path structure; the WDM (10) is provided with a multiplexed port (10 a) and two splitting ports (10 b, 10 c); the corresponding wavelengths of these two splitting ports (10 b, 10 c) are λ1, λ2 respectively; the multiplexed port (10 a) is connected with the end of the sensing optical fiber (6); the splitting port (10 b) is connected with the feedback device (2) and the end of the splitting port (10 c) is no use.
 2. The method according to claim 1, wherein, when the light with wavelength λ1 is transmitting along the sensing optical fiber (6) to the end of the feedback device (2), a part of the light is affected by scatter (reflection) point and return in advance, the remaining light is passing through the WDM (10) and reaching the feedback device (2), and then returning along the same route; therefore, the signal P¹ fromed by wavelength λ1 can be expressed as: $\begin{matrix} {P_{1} = {P_{eff} + {\sum\limits_{i}\; {p_{S}\left( {\lambda_{1},i} \right)}}}} & (1) \end{matrix}$ wherein, P_(eff) is a signal formed by reflecting by the feedback device (2), p_(S)(λ₁, i) is a signal formed by light with wavelength λ1 caused by the i-th scatter (reflection) point of the sensing optical fiber 6, $\sum\limits_{i}\;.$ is the sum of all scatter points along the sensing optical fiber (6) and before the WDM (10). when the light with wavelength λ2 is transmitting along the sensing optical fiber (6) to the end of the feedback device (2), a part of the light is affected by scatter (reflection) point and return in advance, the remaining light is passing through the WDM (10) and leaking from the port (10 c); therefore, the signal P₂ fromed by wavelength λ2 can be expressed as: $\begin{matrix} {P_{2} = {\sum\limits_{i}\; {p_{S}\left( {\lambda_{2},i} \right)}}} & (2) \end{matrix}$ wherein p_(S)(λ₂, i) is a signal formed by light with wavelength λ2 caused by the i-th scatter (reflection) point of the sensing optical fiber (6), using signal processing means to make λ1 close to λ2 and then have: $\begin{matrix} {{p_{S}\left( {\lambda_{1},i} \right)} \approx {p_{S}\left( {\lambda_{2},i} \right)}} & (3) \\ {{\sum\limits_{i}\; {p_{S}\left( {\lambda_{1},i} \right)}} \approx {\sum\limits_{i}\; {p_{S}\left( {\lambda_{2},i} \right)}}} & (4) \end{matrix}$ thus, the interference signal component can be removed and get an effective signal P_(eff).
 3. A system using wavelength division multiplexing for eliminating and reducing light diffusion and light reflection interference in interference path, characterized by: a WDM (10) is series connected between the end of a sensing optical fiber (6) and a feedback device (2) in the conventional interference optical path structure; the WDM (10) is provided with a multiplexed port (10 a) and two splitting ports (10 b, 10 c); the corresponding wavelengths of these two splitting ports (10 b, 10 c) are λ1, λ2 respectively; the multiplexed port (10 a) is connected with the end of the sensing optical fiber (6); the splitting port (10 b) is connected with the feedback device (2) and the end of the splitting port (10 c) is no use.
 4. A system using wavelength division multiplexing for eliminating and reducing light diffusion and light reflection interference in interference path, characterized by: the system is comprising a interference optical path structure including a sensing optical fiber (6) and a feedback device (2); the system is further comprising a WDM (10) series connected between the end of a sensing optical fiber (6) and a feedback device (2); the WDM (10) is provided with a multiplexed port (10 a) and two splitting ports (10 b, 10 c); the corresponding wavelengths of these two splitting ports (10 b, 10 c) are λ1, λ2 respectively; the multiplexed port (10 a) is connected with the end of the sensing optical fiber (6); the splitting port (10 b) is connected with the feedback device (2) and the end of the splitting port (10 c) is no use. 